mine is 3.5"-4" right now and i feed it once a day(krill,cichlid pellets, bloodworms, and rarely shrimp) but a big quantity until its belly looks kind of pregnant, and on weekends i feed it 2x, HOWEVER at this rate my oscar has ONLY grown exactly 2" since i bought it which was 3-3.5 months ago which is quite slow for an oscar, my tank is 55 gallons and to my surprise they say the higher the temp is the faster it grows and the temp is 80-82F. and it has growed slower instead, so yours will probably not grow fast, my oscar is a pig it eats so fast in a blink and can eat to death wont stop begs so much that my severum bited him as it doesnt care if is in front of other fishes begging for food.
So what i am trying to tell you is oscars are not so fast growers as sites say they may grow faster than other cichlids such as severums, etc. but not an inch per month that is in the wild not home aquarium, 1" per month is rare, and as i said before, the conditions are excelent for fast growth and a pig-like-eater like mine and it has barely grown 2.5" in 3-4 months